1 in 6 Americans report a lifetime prevalence of major depressive disorder; however, one-third of depressed patients are resistant to current treatments. The search for more effective pharmacological interventions relies on a better basic science understanding of the pathophysiology underlying mood disorders. The use of animal models, such as chronic social defeat stress, has furthered our understanding of biological mechanisms underlying susceptibility and resilience to stress. Previous studies in our lab showed an increase in excitatory synapses in the nucleus accumbens (NAc), a brain region critical to the reward pathway, in mice susceptible to social defeat. Preliminary data also shows a decrease in inhibitory presynaptic terminals in the NAc following chronic social defeat stress. We hypothesize that downregulation of inhibitory tone in the NAc underlies susceptibility to stress leading to social avoidance behavior. This proposal seeks to more fully examine chronic social defeat stress- induced changes to inhibitory tone in the NAc GABAergic microcircuit and to investigate the contribution of fast- spiking, parvalbumin interneurons to social avoidance behavior.